This invention is concerned with a novel trivalent antimony compound useful as a catalyst in the manufacture of polyesters such as polyethylene terepthalate from dimethyl terepthalate and ethylene glycol. In addition, this invention is also concerned with a novel process for the preparation of said trivalent antimony catalyst.
In the past, many trivalent and pentavalent antimony compounds have been used as catalysts in the production of polyesters such as polyethylene terepthalate. Among these antimony compounds are antimony trioxide, antimony pentoxide, alkyl antimony compounds, aryl antimony compounds and the like. In fact, almost any compound containing antimony can be used as a catalyst for these polyesters. In choosing an antimony catalyst for the commercial production of polyethylene terepthalate however, consideration of cost of the catalyst and the amount of soluble antimony present in the polymerization medium are important factors.
In the commercial manufacture of polyethylene terepthalate usually a mixture of a diester of terephthalic acid such as dimethyl terepthalate and excess ethylene glycol is formed in the presence of a catalyst such as Zn (OAc).sub.2 . 2H.sub.2 O. The mixture is heated to remove an alcohol from the diester with the formation of an ester-exchange product, diglycol terepthalate. An antimony catalyst, such as those mentioned above is then added to the reaction mixture, and the temperature is raised to from 250.degree. to 300.degree.C under reduced pressure to effect polycondensation and form the polyethylene terepthalate.
The amount of soluble antimony provided by the catalyst is an important criterion for judging an effective antimony catalyst. It is often desirable to utilize an antimony catalyst which provides a high degree of soluble antimony in the ethylene glycol reaction medium. This high antimony solubility aids in the activity of the catalyst which in turn causes the reaction to proceed at a faster rate. For example, antimony trioxide which is a frequently used catalyst for the production of polyethylene terepthalate is relatively inexpensive but provides very low antimony solubility in the ethylene glycol reaction medium. Antimony glycoloxide, Sb.sub.2 (OCH.sub.2 CH.sub.2 O).sub.3, another widely used catalyst, is relatively expensive and provides from only 1.5% to 1.7% soluble antimony at 80.degree.C. The antimony catalyst of this invention, however, provides from 5.1% to 5.6% soluble antimony at 80.degree.C which is marked improvement over the prior art catalysts mentioned above making it particularly desirable for commercial production of polyethylene terepthalate. The antimony catalyst of this invention can be produced cheaply by virtue of simple processing steps involving short reaction times and inexpensive starting materials.
It is an object of this invention to describe a novel antimony catalyst for the production of polyethylene terepthalate which provides a high degree of soluble antimony in the polyethylene terepthalate reaction medium.
A still further object is to describe a novel and inexpensive process for the production of said trivalent antimony catalyst.